From http://en.wikipedia.org/wiki/B-roll:

Frames of workprint and of original negative are matched exactly through the use of edge numbers that appeared on each frame of original and work print. But the original was not strung together in a simple linear fashion as was the work print. Instead, the original was edited in a "checkerboard" pattern, with each shot synchronized to an equal length of opaque leader on a second roll. These "A and B" rolls functioned equally to make blind splices, fades, and dissolves possible. Each roll was printed separately onto a single roll of raw stock to produce projection prints.[1] The process is described in the 1982 edition of the "Recommended Procedures" of the Association of Cinema and Video Laboratories, and in the classic text, Film and its techniques.[2]

By the way, this roll contains rather rare footage of women workers doing small parts assembly and inspection in a Detroit Cadillac plant during World War II.

Not that interesting bunch of scenes that show pics of people at a plant putting something together (tanks apparently). Rather bizarre that the pic goes 'out' for minutes at a time.. Wonder what the reason was for that....